Castaños is remembered for his victory over the French under Dupont, whom he surrounded and compelled to surrender at the decisive Battle of Bailen in 1808, where the Napoleonic army was defeated in the open field for the first time and which led to King Joseph having to abandon Madrid at the end of that same month.

[2] He was promoted, in October 1802, to lieutenant general in the same promotion as other future notable Spanish military commanders of the Spanish armies during the Peninsular War, including the Duke of the Infantado, Manuel Lapeña, Juan Carrafa, Juan Pignatelli, Francisco Taranco, Francisco Eguía, and Arturo O'Neill, among others.

[3] His victory over Dupont French troops at Bailen (19 July 1808), in the early stages of the war, was the first time the Napoleonic army had ever been defeated in the open field and led to King Joseph having to abandon Madrid at the end of that month.

[2] On returning to Seville, Castaños was appointed captain general and preparations were made for the Spanish army to enter Madrid mid-August.

[2] Following Napoleon's flight from Elba, and the start of the Hundred Days, Castaños was given command of the Army of Observation of the Right and crossed into Roussillon.